1.6 Given an image represented by an NxN matrix, where each pixel in the image is 4 bytes, write a method to rotate the image by 90 degrees. Can you do this in place?

RotateMatrix.java

public class RotateMatrix {
	public static void main(String args[]){
		int[][] matrix = {
				{1, 2, 3 ,4 ,5 ,6},
				{12, 13, 14, 15, 16, 17},
				{23, 24, 25, 26, 27, 28},
				{34, 35, 36, 37, 38, 39},
				{45, 46, 47, 48, 49, 50},
				{66, 67, 68, 69, 70, 71}
		};
		printMatrix(matrix);
		rotate(matrix, 6);
		System.out.println("after rotate:");
		printMatrix(matrix);
	}
	
	public static void printMatrix(int[][] data){
		int i, j;
		int row = data.length;
		int column = data[0].length;
		
		for(i = 0; i < row; i++){
			for(j = 0; j < column; j++)
				System.out.printf("%3d", data[i][j]);
			System.out.println();
		}
	}
	
	/* 分成n/2 layer进行rotate
	 * 每一层中 又有last-layer个step
	 */
	public static void rotate(int[][] data, int n){
		int step;
		int layer;
		for(layer = 0; layer < n/2; layer++){
			/* last是在这一层中最大的index值
			 * eg. n = 6; 那么layer = 0时， 也就是最开始, last = 5;
			 * 到了第二层 layer = 1; last 就是4了； 
			 */
			int last = n - 1 - layer;
			/* 在每一层Layer中 需要的step也是不同的 最外圈是0到n-2个step 数量是n-1
			 * 到了第二层 就是 0到 n-2-2 了 
			 * 正好是last - layer (画个图会更明白)
			 */
			for(step = 0; step < last - layer ; step++){
				int backup = data[step + layer][layer];
				data[step + layer][layer] = data[last][step + layer];
				data[last][step + layer] = data[last - step][last];
				data[last-step][last] = data[layer][last - step];
				data[layer][last-step] = backup;
			}
		}
	}
}